Abstract: An expandable room structure is provided having relatively movable room portions supported by telescopically sliding tubes, at least one of those tubes being movable also laterally to pivot as the room portion floors slide into the same horizontal plane when the room structure is moving into expanded positions. In retracted positions the room portion floors are vertically spaced to permit one room portion to nest within the other. A rack and pinion drive arrangement is provided to cause relative movement between the tubes. Lateral movement of one of the tubes is permitted by engagement with an inclined surface extending out of the other tube such that the laterally moving tube pivots about the pinion. A ramp surface is formed on the abutting edge of the floor of one room portion, and the abutting edge of the floor of the other room portion moves up and down that ramp portion when transitioning between expanded and retracted positions.